Introduction

Ever been at a garage/boot sale, flea market, or junk shop and seen a weird thing that looks like a cash register and a typewriter mated? These are electromechanical calculators which our dads and granddads used in the office because electronic calculators were far off in the future.

There are many of these now on eBay, but many of them are less than functional. The last time they were used was probably in the 1960s, and all the grease has probably congealed and picked up dust and twigs and things. Also, decades of being moved around and not being serviced means that parts go out of alignment or get broken by the well-meaning and the inquisitive.

Worse, the manuals and repair guides for these machines are either rare or nonexistent.

And so, this disassembly guide will help you take this calculator to pieces so that the pieces can be cleaned, repaired, or replaced. And hopefully you'll learn a bit about the mechanisms that make these machines go.

The specific calculator we're going to be taking apart is the Monroe LA5-160. The LA5-160 is of the L series of Monroe calculators introduced in 1928, with the A meaning Automatic, that is, with an electric motor attached. This model has a 16-digit accumulator and an 8-digit counter, and can multiply and divide by successive addition and subtraction.

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This spring pulls the arms against cams fixed to the digit shafts so that the digit shafts will not rotate when the zero-setting mechanism is not operating. It is important that the digit shafts themselves not rotate, since each digit wheel needs to be able to rotate freely on the shaft.

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Swing the two lock arms that the spring was holding together up and out of the way.

Push the accumulator shaft to the left until you can pull up on the right side. The nut that held the shaft's bushing should be free to remove.

1/4-32 panel nut, width 5/16", height 5/64"

There is a cam lever on the middle shaft. Pull up on the part of it that rests on the cam attached to the left side of the accumulator shaft. You should be able to fully remove the accumulator shaft.

Lifting the cam lever rotates the middle zeroing shaft so that its zeroing teeth move closer to the accumulator shaft. When the accumulator shaft rotates, each digit wheel has a projection that hits up against the zeroing tooth, conveniently stopping that digit wheel at zero.

What would help you probably is a look at the patents for the L. They contain lots of drawings, and if you stare at them long enough and read the text, you can probably figure out how things are supposed to work. Here's my (mostly complete) master list of Monroe patents. Look at the patents filed around 1928 to start with. Hope that helps!

Réparabilité

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